It if it taking a really long time to post a new article in WordPress, there is a good chance you are using WP Smush It! Disable that plugin and it will speed up.

"WP Smush It!" compresses your images so that they load faster for your visitors. This makes it better for them, but if you are uploading large images to your articles, it could take time for that to work.

So you have a choice: use an image editor or compression software yourself to make the images smaller, use "WP Smush It!" and have it take a little longer to post, or just not use it and have larger images on your site.